1. Field of the Invention
The present invention relates to a mobile positioning device, particularly to a mobile positioning device for an MRI (magnetic resonance imaging) inductively coupled coil.
2. Description of the Prior Art
During an MRI data acquisition, a receiving coil is generally used to receive radio frequency signals emitted from a human body, and then the received radio frequency signals are processed for imaging. Referring to FIGS. 1 and 2, the commonly used receiving coils usually have an annular antenna design, in which: an inductor L1 and capacitors Cs and C1 are connected in serial to form a receiving coil 10 for receiving the radio frequency signals. The radio frequency signals are transmitted to a detuned circuit 11 formed by capacitor Cs and an inductor Ls. The operating status of the receiving coil 10 is controlled depending on whether a diode D1 connected with the capacitor Cs and the inductor Ls in parallel is conductive or not, the matching with the system is adjusted via a capacitor Cp connected with the capacitor Cs and the inductor Ls, then the radio frequency signals are transmitted to a plugging connector 30 via a preamplifier Preamp and a choke coil 13; and finally, they are transmitted to the system 40 via said plugging connector 30, and the system 40 is responsible for data processing in said MRI system. FIG. 3 simply shows a way of connection for such receiving coils, the receiving coil 10 is used for receiving the radio frequency signals and is connected to the plugging connector 30, and the radio frequency signals are transmitted to said system 40 via the connection between said plugging connector 30 and the system 40.
The abovementioned receiving coils have the following disadvantages: 1) since one end of the plugging connector 30 is connected to the system 40, and the other end is connected to the receiving coil 10 and the corresponding circuits, a doctor is very much restricted when placing or positioning said receiving coil 10 to receive the radio frequency signals from various body parts of a patient, and it is difficult to move freely; 2) it is easy for the plugging connector 30 to be damaged due to frequent plugging in and pulling out; 3) each receiving coil 10 is connected to a set of a preamplifier Preamp and a choke coil 13, which leads to the costs of the receiving coil being significantly increased, and in extreme cases, the choke coil connected to the receiving coil may cause injury to a patient.
In order to resolve the above problems in the current receiving coils, a design is known for an inductively coupled coil. Referring to FIG. 4, in the practical applications of the above mentioned inductively coupled coils, a receiving coil 10′ is set above a patient's bed 50 for receiving the radio frequency signals, an coupler 20 is divided into two parts, one part is connected to the receiving coil 10′ and this part is called the mobile part 22 of the coupler 20. The mobile part 22 has a coupled coil L2; the other part is fixed below the patient's bed 50 and on an MRI housing, and this part is called the fixed part 24 of the coupled coil and this fixed part comprises an coupled coil L4. After the receiving coil 10′ has received the radio frequency signals, it transmits them to the mobile part 22 of the coupler 20; and through the coupling between the coupled coil L2 of the mobile part 22 and the coupled coil L4 of the fixing part 24, the radio frequency signals are input into the fixing part 24 of the coupler 20, and then they are transmitted into the system 40 for processing. In this way of connection between the above coupled coils, the receiving coil 10′ couples the received radio frequency signals to the system 4 via the coupler 20, so that the plugging connector 30 used in the conventional receiving coil 10 showed in FIG. 1 is eliminated. Therefore, it allows the users to avoid the results due to its damage and the increased costs thereof. Furthermore, doctors can move the receiving coil 10′ freely without restriction since the mobile part 22 is connected to the receiving coil 10′ and is moveable freely with it, rather than that said mobile part 22 being connected with said fixing part 24. The preamplifier and the choke coil connected to the fixing part 24 can be adapted for using with various receiving coils 10′, with significant cost savings, and the patient's safety is assured due to the receiving coil 10′ no longer being connected to the choke coil.
To ensure the normal operation of the inductively coupled coil in the above receiving coil device, one key factor is the coupling coefficient Km between the coupled coils L2 and L4. The larger the value of the Km, the smaller the value of the loss caused by the circuit, and in an ideal case, Km=1, therefore all the signals are coupled into the system without any loss. In order to have an ideal coupling coefficient Km, there are two conditions to meet: 1) it is better if the distance between said coupled coils L2 and L4 is shorter, and in an ideal case, the distance is zero; and 2) it is better if the degree of superposition between said coupled coils L2 and L4 is larger, and in an ideal case, the two of them are parallel to and exactly opposite each other.
However, the coupled coil L2 of the mobile part 22 and the coupled coil L4 of the fixing part 24 of the coupler 20 for the aforementioned known inductively coupled coil can only be effectively coupled within a very small distance, such that the patient's bed 50 cannot be penetrated through effectively. During a coupling process, due to the thickness d1 of the patient's bed 50, the distance between the coupled coil L2 of the mobile part 22 and the coupled coil L4 of the fixing part 24 for said coupler 20 is too large, leading to the coupling coefficient Km being reduced significantly, so the loss of the circuit reaches a unacceptable level. If the mobile part 22 of the coupler 20 is deployed below the patient's bed 50, it is necessary to cut a big groove in the patient's bed 50, and since the mobile part 22 needs to be connected to the receiving coils 10′, it causes difficulties during its operation for inserting in and pulling out of the groove.
Referring to FIG. 5, an improved structure of the inductively coupled coil is proposed in Chinese patent application No. 200610012126.6, in which, an additional couple circuit 26 is fitted between the coupled coil L2 of the mobile part 22 and the coupled coil L4 of the fixing part 24. The additional couple circuit 26 includes coupled coils L2′, L3 which are respectively coupled with the coupled coil L2 of the mobile part 22 and the coupled coil L4 of the fixing part 24. The coupled coils L2′, L3 of said additional couple circuit 26 are fitted above and below the patient's bed 50 respectively and they are connected to each other via a cable. A satisfactory coupling coefficient Km is achieved by coupling the coupled coils L2 and L2′, and the coupled coils L3 and L4 respectively, wherein, the distance between L2 and L2′ is d2, and the distance d3 between the coupled coils L3 and L4 can be set to be very small, such that the coupling between the coupled coils L2 and L4 in the aforementioned known inductively coupled coil is no longer restricted by distance.
The coil disclosed in Chinese patent application No. 200610012126.6 easily meets the first one of the two conditions which are needed for having an ideal coupling coefficient Km mentioned above. During the practical operation of the MRI exam, after having placed the receiving coil, a doctor will move the patient's bed to bring the examinee and the receiving coil into an imaging area. In order to obtain images of various parts, the doctor needs to move the patient's bed freely, so that a specific part to be examined can be positioned in the imaging area. Since the coupled coil L3 is fitted underneath the patient's bed and is moved with the bed, while the L4 is fixed on the surface of the housing, it cannot be ensured that the coupled coil L3 and the coupled coil L4 are completely opposite each other after the patient's bed has been moved freely, therefore the coupling coefficient Km between them is affected, and therefore cannot meet the second one of the two conditions that need to be met for having an ideal coupling coefficient Km.